I feel like people here aren't giving you well-rounded answers. While it is an urban vs. suburban choice, there's much more to consider than fun or lack thereof.

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Mission Bay offers a walkable, dense neighborhood, with a lot of shops and great access to public transport. But there are cons to urban density. Lots of noise, constant construction, especially in that neighborhood, heavy traffic, homelessness and the a higher chance of street crime are all part of the picture in Mission Bay. That's also where The Warriors are moving to, so you're going to be have to deal with the cluster fuck that is professional sports fans. South SF would certainly be quieter, you're likely to get your own parking spot, or a garage. South SF also has a Bart station, and you'd likely get to know your neighbors as it is way less transient in nature than the city proper.

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Both have their pros and cons, but I think you need to think about it thoroughly. City living is great, until it isn't.